It is often necessary, or at least desirable, to concentrate a liquid mixture by removing a portion of the solvent, generally water, from the liquid mixture. The resulting product, therefore, is in a more concentrated form. It has been common to concentrate fruit and vegetable juices such as orange juice, grapefruit juice, grape juice, and tomato juice by evaporation to remove water. In addition, seawater and brackish water have been concentrated by evaporation, although the condensed vapor has been recovered as usable potable water rather than discarded as in concentrating fruit and vegetable juices. Nevertheless, each is a concentrating process. In the case of juice, the concentrate is the desirable product, whereas, in obtaining potable water from seawater or brackish water the concentrate is discarded.
Evaporative concentration as described, as well as evaporation of chemical solutions or liquid dispersions, requires substantial energy since it relies on the latent heat of vaporization. Scaling of equipment and enhanced corrosion are often inherent at the temperatures involved in evaporative concentration. Loss of flavor and aroma also result during evaporative concentration of food products.
Because of the shortcomings involved in evaporative concentration, it has been found advantageous to freeze concentrate many products, particularly those having water as the liquid carrier. Generally, reduced energy is required since freeze concentrating relies on the heat of fusion instead of the heat of evaporation. In such a process, water is removed by first producing ice crystals which are then separated from the concentrate. Next, the ice crystals are washed to remove the remaining concentrate on them. The ice crystals can then be discarded or melted if potable water is desired.
During the wash step, it is difficult to remove the concentrate completely from the ice crystals. When concentrating a product such as orange juice even a small loss of entrained concentrate is quite detrimental economically. See C. Judson King, "Separation Processes", McGraw-Hill, page 725. If the ice crystal size could be increased, removal of the entrained concentrate by washing would be more effective. Lowering the product viscosity would also facilitate washing.
Abraham Ogman's U.S. Pat. No. 4,091,635 discloses an apparatus and method of freeze concentrating an already concentrated feed stream. Ogman employs a two stage system in which each stage uses a freezer-crystallizer and a washer. In the first stage, the concentration is doubled. Ice from the first stage is then brought to the second stage and diluted with a feed stream of low concentration following which the diluted solution is freeze concentrated in the second stage.
In none of the previously disclosed freeze concentration systems presently known to applicant has the liquid mixture being concentrated been recirculated through a freeze exchanger until it reaches the desired concentration. Furthermore, no such recirculation system has been employed to produce ice crystals of relatively large size, and an ice slurry of low viscosity, which can be washed free of remaining concentrate with comparative ease.